464 Messrs. Michelson and Morley on the Wave-length 



June have confirmed the anticipation. The apparatus for 

 observing the interference phenomena is the same as that 

 used in the experiments on the relative motion of the earth 

 and the luminiferous sether. 



Light from the source at s (fig. 1), a sodium-flame, falls on 

 the plane parallel glass a, and is divided, part going to the 

 plane mirror c, and part to the plane mirror b. These two 

 pencils are returned along cae and bae, and the interference 

 of the two is observed in the telescope at e. If the distances 

 ac and ah are made equal, the plane c made parallel with that 

 of the image of b, and the compensating glass d interposed, 

 the interference is at once seen. If the adjustment be exact, 

 the whole field will be dark, since one pencil experiences 

 external reflexion, and the other internal. 



If now b be moved parallel with itself a measured distance 

 by means of the micrometer-screw, the number of alternations 

 of light and darkness is exactly twice the number of wave- 

 lengths in the measured distance ; thus the determination 

 consists absolutely of a measurement of a length and the 

 counting of a number. 



The degree of accuracy depends on the number of wave- 

 lengths which it is possible to count. Fizeau was unable to 



Figs. 1 and 2. 



u 



m 



\S 



observe interference when the difference of path amounted to 

 50,000 wave-lengths. It seemed probable that with a smaller 

 density of sodium vapour this number might be increased, and 



